Compatible four channel fm system

ABSTRACT

A four channel FM system is described. In one embodiment the usual 19 kHz pilot signal is employed to switch between front and rear information, while in another, a 76 kHz switching signal is employed for this purpose.

United States Patent 11 1 Dorren 14 1 Jan. 2,1973

54] COMPATIBLE FOUR CHANNEL FM 3,221,098 11/1965 Feldman ..179/15 BTSYSTEM 1,956,397 4/1934 Nicolson ..179/15 BT [75] lnvntor: Louis Donefilflbrae', 'Zia'l'ifj OTHER PUBLICATIONS Quadrasonics On-the-Air byFeldman, Audio 1 ig Quadracastsystemalnc- Magazine, pp. 22, 23, 24, 78,January 1970. [22] Filed: April @197 RCA Technical Notes, No. 345 ThreeChannel FM Band Stereo System Nov. 1959. PP N04 321989 Quad StereoRoomful of Sound, Consumer Electronics Show Daily, June 28, 1970, pp. 8,18.

Related Apphcauon Data The Quart Broadcasting System by Gerzon, Audio[63] Continuation of Ser. No. 13,902, Feb. 25, 1970, Magazine, pp. 26,27, Sept. 1970.

abandoned.

Primary ExaminerKathleen H. Claffy [52] US. Cl. ..179/15 BT AssistantExaminer--Th0ma,s DAmico [51] Int. Cl .."04'! 5/00 Attorney-Eckhoff andHoppe [58] Field of Search..179/15 ST, 100.1 ST, 100.1 TD

f [57] ABSTRACT [56] Re erences Cited A four channel FM system isdescribed. In one em- UNITED STATES PATENTS bodiment the usual 19 kHzpilot signal is employed to switch between front and rear information,while in 311323233 31132? 33111111:11;""31111111333111131:113$ 2 31other, a 76 switching signal is employed M118 3,176,074 3/1965 Browne..179/15 BT Purpose" 3,375,329 3/1968 Prouty ..179/l00.1 TD 2,870,2481/1959 va1a16n.. ..179/15 RT 5 15 Draw F'gures 3,231,672 1/1966 Collins..179/15 BT 8 40 I9 kHz DOUBLEK 33 kHz AUDIO 92 GA 7155 DOUBLEK l 76 kHz1 1 1 1 l 1 LEFT 12/6117 REAR LEFT FKIJNT PATENTED 2% 3,708,623

sum 2 or 1 5 7 3 3 8 5 5 so 80' w' LEFT 1m 2 KIZHT LEFT- RIGHT RIEHTLEFT RIGHT Z S mow FKONT KEAK REAR 3 S & B(SCA SCA 0 I W 0 1 J1 o 40 o4o 60 so 19 kHz, 67 19 kHz/ 7 PILOT FKEQUENCY PILOT FREQUENCY F I E- 5 II Er E f!E5T EC0N[L Tl1!l[D f0U/BL [80 180 180 a? m R UPPEK SIDEBANDLEFT 58 kHz RIGHT +L0wE/z 510mm) RIGHT mom REAR mom :9 kHz KEAK FIK5T180 5E0N0 180+ kHz I INI ENTOR.

LF RF LR+RR LOU/5 DOKKf/V BY 0 KHZ ATTORNEYS PATENTED A 2M5 3708,6213

' sum 3 0r 5 l3 [7N ENCUDEK ii?) TUNER Z J l 54 [9 kHz TUNED AMP 60v l'L/M/TEK DUI/BLEK 62 38 kHz AMP. 66

E: .TF? KIGHI FK FRONT I INVENTOR.

LOU/5 BOWEN BY fi ATTOKNEV COMPATIBLE FOUR CHANNEL FM SYSTEM CROSSREFERENCE TO RELATED APPLICATIONS:

This application is a continuation in part of my application, Ser. No.13,902 filed Feb. 25, 1970, now abandoned.

SUMMARY OF THE INVENTION A four channel audio system is provided whichis fully compatible with standard FM stereo and mono equipment.According to the present invention, a switching or sampling system isemployed so that four audio channels are transmitted by the FM stationaThese four channels are designated left front, right front, left rearand right rear and are sometimes hereinafter abbreviated LF, RF, LR andRR, respectively.

In accordance with one embodiment of the invention, these four channelsare superimposed on the 38 kHz subcarrier and the usual 19 kHz pilotsignal is used as a switching signal. During the first half cycle of the19 kHz signal, the left front and right front information is transmittedwhile during the second half cycle, the left rear and right rearinformation is transmitted. The 19 kHz pilot is then used as a switchingsignal between the front and rear information as is hereinafterdescribed in detail while the 38 kHz signal switches between left andright in the usual manner. This system has the advantage of notrequiring an increase in bandwidth, not requiring any additional pilotor subcarrier frequencies and permits the radio station to continue touse its normal 67 kHz subcarrier for SCA purposes. However, thisembodiment does not ordinarilypermit the use of the full frequencyspectrum for front and back information so that it is sometimespreferable to provide another subcarrier to carry the front and rearinformation. When this is done, the 19' kHz pilot signal is quadrupledand this quadrupled signal is used for switching between front andbackinformation.

Both embodiments of the present invention are completely compatible withpresent mono and stereo equipment. The main channel carries all fouraudio channels so that on a mono receiver the four audio signals arecombined. On a stereo receiver, the left and the right information isextracted in the usual manner and it is only with the receiver equippedfor four channel reception that the. signal producedby the system of thepresent invention is broken into its four components. The systemutilizing the 76 kHz switching signal has the additional. advantage thatthere is a complete reproduction of the full audio bandwidth by each ofthe four channels.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of atransmitter embodying the present invention.

FIG. 2 is a block diagram of a receiver embodying the present invention.

FIG. 3 is an analogy diagram of one of the encoders shown in FIG. 1.

FIG. 4 is a similar diagram of one of the decoders utilized in FIG. 2.

FIG. 5 is a spectrum diagram of the signal employed during thetransmission of front channel information.

FIG. 6 is a similar diagram showing the signal employed during thetransmission of rear channel information.

FIG. 7 is a diagram of the composite wave form employed.

FIG. 8 is a block diagram showing how combiner net-.

ployed.

FIG. 10 is a wave form of that embodiment of the invention wherein a 76kHz subcarrier is employed.

FIG. 11 is a band distribution diagram of the system using the 76 kHzsubcarrier.

FIG. 12 is a block diagram of the encoder employed with the 76 kHzsubcarrier system.

FIG. 13 is a block diagram of the receiver employed with the system.

FIG. 14 is a switch analogy for the transmitter. FIG. 15 is a switchanalogy for the receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is made to FIGS. 1through 9 which illustrate that embodiment of the invention employing al9 kHz switching signal.

At the 'present time, the authorized stereophonic system used in theUnited States includes a multiplex signal wherein left and right channelinformation is carried on a single carrier wave. The composite signalincludes a main channel signal on the carrier frequency which containsboth the right and left signals. A suppressed carrier double side bandsignalis provided on a subcarrier at 38 kHz which carries the left minusright signal. A 19 kHz pilot signal is provided for phase lock of the 38kHz side bands. In addition, there may be a subsidiary communicationauthorization (S.C.A.) signal at 67 kHz and this system does notinterfere with the S.C.A. signal. In general, this embodiment of theinvention is carried out by employing the 19 kHz pilot signal as aswitching signal to switch between front and back information to providefor front and back as well as left and right signals.

The method of transmitting the signal is shown in FIGS. 1 and 3. Astandard multiplex exciter 8 is employed, and this generates a 19 kHzpilot signal which serves as the switching signal for the front and rearinformation. This signal is passed through lead 10 to diodes 12 and 14and alternate half cycles are passed by leads l3 and l5'to the encodersl7 and 19. One of these encoders is shown in analog form in FIG. 3 andconsists of switches 16 and 18. The encoder has a line 20 leading toswitch 16 for front information and a line 22 leading to switch 18 forrear information and a common output line 24. The switches are actuatedby pulses from lines 13 and 15 respectively. Thus, on a positive halfcycle of the 19 kHz signal, switch 16 is closed so that the frontinformation is fed through line 24 while on the following negative halfcycle, switch 18 is closed so that the rear information is fed throughline 24. The other encoder is not described since it operates in exactlythe same way with the right front and rear information. The leftinformation fed through line 24 to the multiplex exciter 8 while thecorresponding right channel information is fed through line 26 to theexciter. The signal generated by the exciter is then fed to thetransmitter 28.

The signal thus generated can best be understood by reference to FIGS. 5and 6. FIG. 5 represents the signal as it is being transmitted duringthe first or positive half cycle of the 19 kHz signal. It will be seenthat at frequency (zero in this sense represents the nominal carrierfrequency of the FM station) there is present up to l kHz, a signalrepresenting the left front plus right front information. Centered on 38kHz is the left front minus right front information. In FIG. 6 thesignal is shown during the second or negative half cycle of the 19 kHzsignal. During this half cycle, the main channel is carrying the leftplus right information but in this case it is the rear information whilesimilarly centered on 38 kHz the left minus right information is sentbut here again it is the rear information. It will be seen that in bothinstances, there is no interference with the normal S.C.A. signal.

The method of receiving a signal is shown in FIGS. 2 and 4. The signalis received on an ordinary FM tuner 30 and fed to a standard multiplexdemodulator 32. The 19 kHz signal is taken from line 34 and passed todiodes 36 and 38 and the rectified 19 kHz pulses are passed throughlines 40 and 42 to the decoders 44 and 46. The left and right channelinformation is taken from the demodulator 32 and passed through lines 48and 50 to the respective decoders. Referring now specifically to FIG. 4,the upper switch is actuated by the positive half cycles from line 40while the lower switch is operated by the negative half cycles from line42 so that the decoder switches between the left front and the rightfront on positive and negative pulses of the 19 kHz signal. The signalsare then taken from the decoder and amplified in the usual way. At thesame time, the right channel information is handled in the same mannerby the decoder 46.

In order to prevent high frequency components of the incoming programinformation from interfering with operation of the system, low passfilters which pass only frequencies below kHz are employed in the inputleads to the encoders, i.e., leads and 22 of encoder 17 and thecorresponding leads of the right hand encoder 19. Similar low passfilters are also employed in the output leads of decoders 44 and 46.

The operation of the overall system can best be seen in FIG. 7 whereinit is assumed that demodulation takes place on a time division basisalthough it will be obvious that the invention is equally applicable todemodulators which operate on a matrixin'g system or a combination oftime division and matrixing. FIG. 7

. represents one complete cycle of the 19 kHz signal and two completecycles of the 38 kHz subcarrier. During the first half of the 19 kHzcycle, front information exclusively is being sent both on the mainchannel and on the 38 kHz subcarrier. In the case of the main channel,this is a combination of the left front and right front informationwhile on the 38 kHz subcarrier, left front information is being sent onthe upper side band and right front information on the lower side band.During the next half cycle of the 19 kHz signal, the situation isreversed with the main channel carrying left rear plus right rearinformation while on the 38 kHz signal, left rear information is carriedon the upper side hand while right rear information is carried on thelower side band.

Although the embodiment heretofore described is a fully workable system,some modifications can be made for optimum results.

When the full bandwidth of 15 kHz is transmitted with this signal, a 15kHz audio tone will produce sidebands of 4 kHz and 34 kHz when imposedon the 19 kHz pilot signal. The 34 kHz signal will beat with sidebandcomponents of the 38 kHz subchannel causing beat frequencies in the suband main channels, and the 4 kHz signal will beat with the main channelaudio components, creating beat frequencies lying within the audiblerange. Thus, to employ the full 15 kHz bandwidth, it is desirable toprovide a combining network on the inputs to the encoder as is shown inFIG. 8. Here the left channel encoder 17 having the inputs 20 and 22,previously described, has a combining network 52 connected between theinput lines. Obviously the right channel is treated the same way. Thiscombining network combines all frequencies above 4 kI-Iz so that for thehigher frequencies, the information is carried on both the front and therear channels. At frequencies under 4 kHz there is a separation betweenfront and back. This gives very good presence since it has been foundthat a great deal of the separation presence occurs below 4 kHz.

With some relatively inexpensive FM receivers, some distortion may beencountered for the reason that such receivers employ a 19 kHz tunedamplifier together with a doubler. The 19 kHz switching signal in thecase of such receiver is not a clean 19 kHz but has certain modulationcomponents imposed thereon. In other words, the 19 kHz switching signalcontains amplitude modulated components so that switching does notalways take place at the exact points desired. In order to remedy this,a limiter as is shown in FIG. 9 may be employed with such receivers.Here the tuner 54 feeds a signal through line 56 to the audio gates 58and at the same time the signal is fed to the 19 kHz tuned amplifier 60.The 19 kHz signal is now passed to the limiter and doubler 62. Thelimited 19 kHz signal is passed through line 64 to the audio gates toperform the switching function and, since it is now free of allamplituded modulated components, provides a clean switching action. Thedoubled signal is fed to the 38 kHz amplifier 66 which serves as theswitching signal for the right and left information in the usual manner.Most FM receivers do not require this added circuitry, particularly thebetter grade of receivers which use a phase locked oscillator ratherthan the simple tuned amplifier and doubler.

The same basic system is used in the scheme shown in FIGS. 10 through 15except here instead of using the 19 kHz signal to switch between frontand rear information, the 19 kHz signal is first doubled in the usualmanner to act as a switching signal for the left and right informationand again doubled to produce a 76 kHz signal which serves to switch thefront and rear information. In order to preserve compatibility, theorder in which. theaudio signals are transmitted is changed to LR, LF,RR and RF. It is also necessary to make a change in the bandwidth tohandle the system and in the specific system described, this must be atleast 91 kHz for the four channel transmission and it may be increasedto l kHz to handle S.C.A. subcarrier.

In FIG. 11, one half of the composite signal is shown. Thus, there is amain channel in the usual manner extending from 50 Hz to kHz and thiscontains the left plus right information, both front and rear. There isa pilot signal at 19 kHz and a first subchannel centered on 38 kHz. Thisfirst subchannel contains the left minus right information, includingboth front and rear. A second subchannel is centered on 76 kHz and thiscontains the front minus rear information. Summingup the above, it canbe seen that the novel composite signal of the present inventionincludes the following:

A main channel extending up to 15 kHz and including the sum of thesignals, for example, left and right both front and rear.

A 19 kHz pilot signal. A first sub-channel centered on 38 kHz containingleft minus right information.

A sub-channel centered on 76 kHz containing front minus rearinformation.

If this signal is studied, purely mathematical Fourier analysis showsthat the following signal equations actually exist. The first of theseequations is assigned to the main channel and is the sum of the signals,i.e., (Lf-i-Lr-l-l-Rr); this main channel extending up to 15 kHz. Thesecond equation is (Lf-I-Lr-R Rr) and is located in the firstsub-channel which is centered at 38 kHz. Also located in the firstsub-channel is another equation (LfLrRf+Rr). To differentiate these twoequations, they are modulated in quadrature; that is, the first beingthe sine of 38 kHz and the second being the cosine of 38 kHz. The finalequation is centered at 76 kHz is (LfLr+Rf-Rr).

With the production of these four equations in the base band signal, wehave now satisfied the algebraic conditions of transmission. The purelymathematical Fourier analysis is as follows:

ANALYTICAL DESCRIPTION OF THE DORREN QUADRAPLEX COMPOSITE SIGNALANALYSIS: THE MODULATING FUNCTION OF FOUR CHANNELS The modulatingfunctions of four channels are assumed to vary in the ways shown below.

The origin of the time scale is chosen as the beginning of one of thesampling pulses:

si (t):

si (t) is a periodic function with a fundamental frequency 2f and has afourier series representation;

cosin 411' nft 2 4/n1r l cosin n1r/2) Retaining only the 38 kHz and 76kHz components and applying the relations of one to generate the otherthree functions, we get:

s1 (t) E4 {1 4/1rcosine 41rft 4/1rsine 41rft 4/11 sine 81rft.}

s2 (t) 13/4 {1 4/1rcosine 41rft +4/1r sine 41rft 4/11- sine 8'n'ft.}

s3 (1) 15/4 {I 4/1rcosine 41rft 4/1rsine 41rft 4/11- sine 81rft.}

s4 (1) E/4 {l 4/11- cosine 41rft 4/11 sine 41rft 4/11- sine 81rft.}Multiplying si (t) by ai (t) and summing to give the composite, we get:

To this signal a pilot should be added of the form A sin 21rft. We seethat the quadraplex composite signal consists of:

l. a main channel component (al a2 +.a3 a4).

2. two 38 kHz components in quadrature, one modulated by (al a2 a3 a4)and the other modulated by (al a2 a3 +a4).

3. one 76 kHz component modulated by (al a2 If we make the followingchannel identification,

al left front signal a2 left rear signal a3 right front signal a4 rightrear signal and assume the two channel stereo case in which,

a1 a2 left a3 a4 right the composite signal reduces to c (t) E/4 (21 2r)A sin 21rft (21 2r) sin 41rft. This is the standard two channel stereoformat with the pilot at 19 kHz and having the correct phaserelationship to the 38 kHz subcarrier.

This composite signal is generated by the transmitter circuit as shownin FIG. 12 and theswitch analogy as shown in FIG. 14 wherein the switchanalogy switches at the rates of 76 kHz and 38 kHz.

If an S.C.A. signal is desired, this can be centered on 105kI-Iz,'although the provision for such a signal forms no part of thepresent invention.

In FIG. 10, the sampling system is shown. There is shown one full cycleof the 38 kHz subcarrier and natu-. rally two cycles of the newlygenerated 76 kHz subcarrier. The 38 kHz signal is utilized by astereophonic receiver in the usual manner so that the sampling pointswould be at points 68 and 70 for the left and right handed information.Similarly, the signal centered on the 76 kHz subcarrier contains thefront minus rear infonnation so that on a four channel audio system thesampling points would be at 72, 74, 76 and 78 to extract the desiredinformation.

In FIG. 13 there is shown a block diagram of how the decoder works. Thecomposite signal comes from the tuner through line 80 and a portion ofthe composite signal goes to the 19 kHz amplifier 82. The compositesignal is also fed to the audio gates 84. The 19 kHz signal is doubledto 38 kHz in the doubler 86 and this signal is passed through line 88 tothe audio gate 84 where it is used to switch between the right and leftinformation. A portion of the signal is also sent to the second doubler90 which puts out a signal at 76 kHz through line 92 which is also sentto the audio gates and utilized to switch between front and backinformation.

The transmitter circuit is shown in FIG. 12 and essentially consists ofthe opposite circuit from that described for the decoder. Thus, foursources of audio are supplied through the four lines 94 A, B, C and D tothe audio gates 96. A 76 kHz oscillator 98 is provided which sends asignal to the audio gates for switching between the front and the backinformation. The signal is divided by two in divider 100 and a portionof this 38 kHz signal is sent to the audio gates for switching betweenright and left information while a portion of this signal is sent to thedivider 102 for-the generation of the 19 kHz pilot signal. The pilotsignal is combined with the composite signal from the audio gates toproduce a composite signal on 104 which can be used to modulate astandard FM transmitter. Naturally this signal will look like the signalof FIG. 11 except that no description has been included of thegeneration of the SCA band.

FIG. 14 shows a mechanical switch analogy of the switching circuit andthis as well as FIG. should be utilized in conjunction with FIG. 10.Here switch 104 operates at a frequency of 38 kHz for sampling the rightand left information while switch 106 operates at twice this frequencyfor sampling the left rear and left front information while switch 108operates at the same frequency for the same purpose in the rightchannel. Thus one can visualize switch 104 in the upper position whileswitches 106 and 108 are also in the upper position. Switch 108 is ineffect inoperative since the right channel is open but switch 106switches the left rear information into the outgoing signal. Now switch106 (as well as 108) moves to the lower position so that the left frontinformation is sampled. After one complete cycle of the 76 kHz, switch104 moves to the lower position while switch 108 repeats the operationfor sampling the right front and rear information. FIG. 15 gives asimilar analogy for the receiver where switch 110 operating at afrequency of 38 kHz switches between right and left information whileswitches 112 and 114 similarly switch between front and rearinformation. Naturally, these are only mechanical analogies and in anormal receiver or transmitter such switching is by solid state devices.

It will be apparent from this description that the signal is completelycompatible with either a mono, stereo, or four channel receiver. Thus,on a mono receiver, one would hear the main channel which during twocycles of the 76 kHz subcarrier will contain the information from allfour channels. On a stereo receiver, left front and left rearinformation will be extracted during the first 180 period of the 38 kHzsubcarrier while the right front and right rear information will bereceived during the second period. On the four channel receiver, thefour signals would be individually received as previously described.

It will be seen from the description which has been given that thecomplete signal of the first embodiment has been contained within theassigned bandwidth and that there has been no interference with anS.C.A. signal, if this is being sent. In the second embodiment the foursignals are all modulated to the full 15 kHz bandwidth so that there isno deterioration of separation over this bandwidth. The four signals arealso given the same percentage of modulation so there is nodeterioration of the signal to noise ratio.

1 I claim:

l. A compatible four channel audio system for use in conjunction with anF. M. radio transmitter having a composite signal modulated on thecarrier thereof consisting of a main channel, a 19 kHz synchronizingpilot signal, a first sub-channel which is the second harmonic of saidpilot signal and a second sub-channel which is the fourth harmonic ofsaid pilot signal, wherein said composite signal is generated byproviding first, second, third and fourth audio signals, providing a 76kHz sub-carrier frequency and utilizing said 76 kHz frequency to switcha first audio gate between the first and second audio signals and asecond audio gate to switch between said third and fourth audio signals,providing a 38 kHz sub-carrier frequency and utilizing said 38 kHzfrequency to switch a third audio gate between the outputs of the saidfirst two audio gates, providing a pilot signal of 19 kHz and combiningsaid pilot signal with the output of said third audio gate and utilizingthis composite signal to modulate an F .M. transmitter.

2. A decoder for demodulating the four channel composite signal asgenerated by the system of claim 1, comprising in combination:

a. means for extracting the 19 kHz pilot signal,

b. means for providing a first switching frequency at twice said 19 kHzfrequency,

c. first audio gate means for switching said composite signal at saidfirst switching frequency into two signals,

. means for providing a second switching frequency at twice said firstswitching frequency, and

0. second audio gate means for switching each of said two signalsgenerated by said first audio gate means at said second switchingfrequency to provide four audio output signals.

3. A four channel audio system for use in conjunction with an F. M.radio transmitter for transmitting a composite signal which compositesignal includes a main channel with first, plus second, plus third andplus' fourth audio signals combined thereon, a pilot signal removed fromsaid main channel, a first suppressed sub-channel which is the secondharmonic of said pilot signal, said first sub-channel having first, plussecond,.

minus third, minus fourth audio signals and in quadrature therewithfirst, minus second, minus third and plus fourth audio signals modulatedthereon and a second suppressed sub-channel centered on a frequencytwice that of said first sub-channel having first, minus second, plusthird, minus fourth audio signals thereon, means for doubling said pilotsignal and means for utilizing said doubled pilot signal to switchbetween first plus second and third plus fourth audio signals to obtaintwo signals, quadrupling said pilot signal and utilizing said quadrupledpilot signal to switch between the first and second audio signals andbetween the third and fourth audio signals to obtain four audio signals.

4. The four channel audio system, of claim 3 wherein the F.M. signalcontains the following components:

a. a main channel extending up to kHz having first, plus second, plusthird, plus fourth audio signals,

b. a 19 kHz pilot signal,

0. a 38 kHz suppressed sub-channel having first, plus second, minusthird, minus fourth audio signals and in quadrature therewith first,minus second, minus third, and plus fourth audio signals modulatedthereon,

d. a 76 kHz suppressed sub-channel having first, minus second, plusthird, minus fourth audio signals thereon,

e. means to provide a 38 kHz signal and to utilize said signal to switchbetween said first plus second and third plusfourth audio signals, and gf. means to provide a 76 kHz signal and to utilize said signal to switchbetween said first and second audio signals and between said third andfourth audio signals.

5. A four channel RM. system which is fully compatible with existingmono and stereo standards wherein there is employed a composite signalhaving a main channel extending to 15 kl-lz'from the carrier frequencyof an F.M. transmitter with plus first, plus second, plus third and plusfourth audio signals combined thereon, a 19 kHz pilot signal removedfrom said carrier frequency, a first sub-channel which is the secondharmonic of said pilot signal, said first subchannel containing twosub-carriers, the first modulated with the audio signal: plus first,plus second minus third minus fourth audio signals and in quadraturetherewith the second subcarrier modulated with the audio signal: plusfirst, minus second, minus third and plus fourth audio signals and asecond sub-channel centered on the fourth harmonic of said pilot signalcontaining a sub-carrier modulated with the audio signal: plus first,minussecond, plus third and minus fourth audio signals. 1

1. A compatible four channel audio system for use in conjunction with anF. M. radio transmitter having a composite signal modulated on thecarrier thereof consisting of a main channel, a 19 kHz synchronizingpilot signal, a first sub-channel which is the second harmonic of saidpilot signal and a second sub-channel which is the fourth harmonic ofsaid pilot signal, wherein said composite signal is generated byproviding first, second, third and fourth audio signals, providing a 76kHz sub-carrier frequency and utilizing said 76 kHz frequency to switcha first audio gate between the first and second audio signals and asecond audio gate to switch between said third and fourth audio signals,providing a 38 kHz sub-carrier frequency and utilizing said 38 kHzfrequency to switch a third audio gate between the outputs of the saidfirst two audio gates, providing a pilot signal of 19 kHz and combiningsaid pilot signal with the output of said third audio gate and utilizingthis composite signal to modulate an F.M. transmitter.
 2. A decoder fordemodulating the four channel composite signal as generated by thesystem of claim 1, comprising in combination: a. means for extractingthe 19 kHz pilot signal, b. means for providing a first switchingfrequency at twice said 19 kHz frequency, c. first audio gate means forswitching said composite signal at said first switching frequency intotwo signals, d. means for providing a second switching frequency attwice said first switching frequency, and c. second audio gate means forswitching each of said two signals generated by said first audio gatemeans at said second switching frequency to provide four audio outputsignals.
 3. A four channel audio system for use in conjunction with anF. M. radio transmitter for transmitting a composite signal whichcomposite signal includes a main channel with first, plus second, plusthird and plus fourth audio signals combined thereon, a pilot signalremoved from said main channel, a first suppressed sub-channel which isthe second harmonic of said pilot signal, said first sub-channel havingfirst, plus second, minus third, minus fourth audio signals and inquadrature therewith first, minus second, minus third and plus fourthaudio signals modulated thereon and a second suppressed sub-channelcentered on a frequency twice that of said first sub-channel havingfirst, minus second, plus third, minus fourth audio signals thereon,means for doubling said pilot signal and means for utilizing saiddoubled pilot signal to switch between first plus second and third plusfourth audio signals to obtain two signals, quadrupling said pilotsignal and utilizing said quadrupled pilot signal to switch between thefirst and second audio signals and between the third and fourth audiosignals to obtain four audio signals.
 4. The four channel audio systemof claim 3 wherein the F.M. signal contains the following components: a.a main channel extending up to 15 kHz having first, plus second, plusthird, plus fourth audio signals, b. a 19 kHz pilot signal, c. a 38 kHzsuppressed sub-channel having first, plus second, minus third, minusfourth audio signals and in quadrature therewith first, minus second,minus third, and plus fourth audio signals modulated thereon, d. a 76kHz suppressed sub-channel having first, minus second, plus third, minusfourth audio signals thereon, e. means to provide a 38 kHz signal and toutilize said signal to switch between said first plus second and thirdplus fourth audio signals, and f. means to provide a 76 kHz signal andto utilize said signal to switch between said first and second audiosignals and between said third and fourth audio signals.
 5. A fourchannel F.M. system which is fully compatible with existing mono andstereo standards wherein there is employed a composite signal having amain channel extending to 15 kHz from the carrier frequency of an F.M.transmitter with plus first, plus second, plus third and plus fourthaudio signals combined thereon, a 19 kHz pilot signal removed from saidcarrier frequency, a first sub-channel which is the second harmonic ofsaid pilot signal, said first sub-channel containing two sub-carriers,the first modulated with the audio signal: plus first, plus second minusthird minus fourth audio signals and in quadrature therewith the secondsubcarrier modulated with the audio signal: plus first, minus second,minus third and plus fourth audio signals and a second sub-channelcentered on the fourth harmonic of said pilot signal containing asub-carrier modulated with the audio signal: plus first, minus second,plus third and minus fourth audio signals.